How to accept counteroffer?

I recently got a new job, the agreement was that I work one day of the week and a full time position may be available in the future. Well, after only working one shift and just recently getting hired, the company is offering me full time. I haven’t quit my other job yet, and my boss gave me a counteroffer which is a raise and fulltime schedule. I want to accept the counteroffer, but don’t know how to tell that to my new job. I feel bad about basically quitting after only working one shift, but I’d be stupid to turn down a raise.

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